Spectroscopic study of nanocrystalline V2O5•nH2O films doped with Li ions

Optical properties of nanocrystalline, LixV2O5 center dot nH(2)O films (0<x<22 mol%), are explored in the present work. These films have been produced by the sol-gel technique (colloidal route), which was used for the preparation of high purity and homogeneity films. Optical measurements were carried out using a double-beam spectrophotometer. The optical constants such as refractive index n, the extinction coefficient k, absorption coefficient a, and optical band gap of the films material have been evaluated. The optical absorption coefficient was calculated from the measured normal reflectance, R, and transmittance, T, spectra. The optical spectra of all samples exhibited two distinct regions: at high energy, which suggests a direct forbidden transition with optical gap ranging from 1.75 to 2.0 eV and increases with increase in Li-content. On the other hand a second low-energy band suggests a direct allowed transition with optical gap ranging from 0.40 to 0.42 eV. The width of the localized states (band tail) E-e was also estimated for all samples. Additional calculations applying the real part of the optical dielectric function led to the evaluation of the charge carrier concentration and their effective mass. (C) 2010 Elsevier Ltd. All rights reserved.